1. Field
Aspects of the present disclosure relate generally to wireless communication systems, and more particularly, to call handover in wireless communication environments.
2. Background
Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. One example of such a network is the UMTS Terrestrial Radio Access Network (UTRAN). The UTRAN is the radio access network (RAN) defined as a part of the Universal Mobile Telecommunications System (UMTS), a third generation (3G) mobile phone technology supported by the 3rd Generation Partnership Project (3GPP). The UMTS, which is the successor to Global System for Mobile Communications (GSM) technologies, currently supports various air interface standards, such as Wideband-Code Division Multiple Access (WCDMA), Time Division-Code Division Multiple Access (TD-CDMA), and Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). The UMTS also supports enhanced 3G data communications protocols, such as High Speed Packet Access (HSPA), which provides higher data transfer speeds and capacity to associated UMTS networks.
As the demand for mobile broadband access continues to increase, research and development continue to advance the UMTS technologies not only to meet the growing demand for mobile broadband access, but to advance and enhance the user experience with mobile communications.
In legacy wireless systems, a user equipment (UE) and/or one or more network devices may be configured to handover a call of the UE from a serving cell to a target cell, for example, when signal quality associated with the serving cell becomes degraded. To successfully perform a handover operation between cells, signal timing between the UE and the target cell must first be synchronized. Furthermore, one parameter that is relevant to the signal synchronization process is the unique system frame number (SFN) associated with the target cell, which is a cell-level synchronization parameter that coordinates signal timing between network entities in the target cell and the UEs that these network entities serve. The SFN is also used for establishment of a dedicated physical channel (DCH) between the UE and a target cell during handover. In some handover scenarios, however, UEs are initially unaware of the SFN associated with a target cell before handover operations commence and must demodulate and process a target cell primary common control physical channel (P-CCPCH) to decipher the target cell SFN in order to proceed with the handover.
One such scenario in which a UE may be initially unaware of a target cell SFN may be in inter-radio access technology (RAT) handover. For example, in some wireless communications environments, multiple cells utilizing different RATs may exist, such as those conforming to technology standards such as Long Term Evolution (LTE), Global System for Mobile Communications (GSM) (e.g. Enhanced Data Rates for GSM Evolution (EDGE) and/or Global Packet Radio Service (GPRS)), or WCDMA. Though UEs operating in such wireless communication environments may prefer to utilize cells associated with one of these RATs relative to other RATs, the UE may be programmed to hand service over to a cell utilizing a different RAT where the service associated with the preferred RAT becomes degraded. For example, some UEs are configured to utilize 4G or GSM cells when possible, but may be handed over to a cell utilizing WCDMA where the 4G or GSM service deteriorates. Such inter-RAT handover may preserve an ongoing call where the preferred RAT can no longer maintain the call. The UE, however, is not initially aware of the SFN of the target cell, and therefore must demodulate and process the target cell P-CCPCH to decipher the SFN in order to correctly establish a DPCH and otherwise proceed with the handover.
Furthermore, UEs may also be unaware of a target cell SFN in certain handover scenarios involving a serving cell and a target cell that utilize the same RAT. For example, where the UE is in a connected state (e.g. CELL_FACH) on a first frequency on a source cell utilizing WCDMA and the network requires the UE to move to dedicated mode (e.g. CELL_DCH) on a second frequency on a target cell also utilizing WCDMA, the UE may be initially unaware of the SFN associated with the target cell. Again, in this intra-RAT handover scenario, the UE must first receive, demodulate, and process the target cell P-CCPCH to obtain the SFN before it can establish the required DPCH with the target cell.
Therefore, in these and other handover scenarios wherein the UE is unaware of the target cell SFN before initiation of handover procedures, the UE must determine the SFN of the target cell and use at least this SFN to establish a DCCH with the target cell. In legacy devices and systems, these and other handover operations are performed in series. For example, the UE may acquire the WCDMA cell, decode the SFN associated with the cell by detecting and decoding the P-CCPCH transmitted by the cell, compute the connection frame number (CFN) from the SFN, establish a downlink dedicated physical channel (DL-DPCH), perform a synchronization procedure (e.g. SyncA procedure), and establish an uplink dedicated physical channel (UL-DPCH)—performing all of these operations in a linear sequence.
Recently, consumer and network operator demand for faster and more reliable cell handover has increased significantly. Performing the foregoing handover operations according to current procedures limits the ability for networks and UEs to meet these handover time demands. Therefore, method and apparatus for increasing the efficiency of handover are needed.